Visual pigment regeneration is an absolute requirement for maintenance of visual sensitivity in the mammalian retina. However, the relationship between pigment regeneration and the recovery of sensitivity following exposure to bright light (dark adaptation) is presently poorly understood. The elucidation of these fundamental mechanisms is of great importance, as the impairment of pigment regeneration and the resulting slowing of dark-adaptation in photoreceptor cells are hallmarks of a number of blinding eye diseases, including Oguchi's disease, Stargardt's disease, Leber's congential amaurosis, fundus albipunctatis, and age-related maculopathy. The overall aims are to determine the cellular mechanism(s) within rod photoreceptors isolated from the retinal pigment epithelium that regulate the rate of pigment regeneration and dark adaptation in mammalian rods, processes that are substantially slower than in mammalian cones. Recovery of sensitivity will be measured electrophysiologically, the reduction of all-trans retinal to all-trans retinol following pigment bleaching will be measured microfluorometrically, the rate of pigment regeneration will be measured microspectrophotometrically. This multidisciplinary approach will be applied to wild-type (WT) and transgenic mouse rods before and following pigment bleaching, and after exogenous treatment with 11-cis retinal to regenerate visual pigment. Two Aims are proposed: In Aim I we will determine the cellular mechanisms that regulate the rate of visual pigment regeneration in rod photoreceptors isolated from the retinal pigment epithelium following their exposure to bright light. We will test the hypotheses that the decay of long-lived photoproducts of bleaching (Meta III) limits the rate of pigment regeneration, that during regeneration 11-cis retinal partitions into intracellular disks of bleached rods from lipid binding partners in the extracellular space surrounding rods by a passive diffusion, and that the residence of opsin in intracellular disks in rods slows pigment regeneration rate compared to cones in which opsin is located in the plasma membrane. In Aim II we will determine and characterize the time course of mechanisms by which regenerated visual pigment leads to recovery of visual sensitivity in rod photoreceptors Together with our collaborator, Dr. Jeannie Chen,we will test the hypotheses that recovery of sensitivity in bleached and regenerated mouse rods occurs in temporally and mechanistically distinct stages that are associated with dephosphorylation of C-terminal residues on regenerated rhodopsin and/or transducin translocation. The knowledge we will gain is prerequisite for understanding dark adaptation under normal and disease-related conditions, and is required for development of novel therapies to treat blinding eye conditions.